Typewriter platens are rigidly mounted in tightly fixed spacial positions so that the printing mechanism will impact the surface of the platen, and the printed sheet carried thereby, with an equal force at both ends of the relative travel of the printing mechanism with respect to the platen while permitting rotation of the platen. A significant problem in typewriter manufacture is the insuring of even, smoothly fed paper sheets. If there is an uneveness in the force exerted by the feed rolls against the platen or against the paper being fed around the platen, the paper will wrinkle and if the axis of the feed rolls is not parallel with that of the platen the effect will be to cause the paper to deviate from a straight feed path or skew and will work itself either to the left or right causing the printed lines to not correspond with a horizontal print line across the paper.
In order to insure that the feed rolls are positioned parallel to the axis of the platen and that the feed rolls exert equal forces against the platen, extensive adjustments and design effort must be made. It is conventional to support a bracket at a pivot point, which bracket in turn supports the end points of the axles upon which the feed rolls are mounted, thereby equalizing the forces between the front and rear feed rolls. The front to rear placement of the supports for the feed rolls has been traditionally through the tightening and loosening of bolts through lost motion slots or adjustment slots in the brackets, or tightly defined shaft support positions in the frame. If the parts are slightly out of proper fit the different pressures on the rollers will cause an improper feeding and must be adjusted by a trial and error technique.
Prior art feed roll devices by virtue of their design can only address one of the two significant problems. Either the feed roll must be supported in such a manner that it allows an automatic alignment of the roll with the platen such as that technique disclosed in U.S. Pat. No. 2,218,108 to Harmon wherein the feed roll support bracket or truck is permitted to pivot about a central point within a restricted plane, or the addressing of the problems of feed roll engagement force against the paper and platen by the spring biasing of the feed rolls by exertion of the spring force onto a portion of linkages which in turn support the feed roll end brackets.
Examples of prior art where the pivot points of the linkages will define the arc through which the feed rolls support brackets will move include Bower, U.S. Pat. No. 2,121,853; Martin et al. U.S. Pat. No. 2,297,490; Armstrong U.S. Pat. No. 1,453,582; Garbell U.S. Pat. No. 1,841,116 and Gerring U.S. Pat. No. 1,910,329. In each instance the pivot points of the support members must be adjusted or fabricated in the overall framework of the typewriter to insure exact parallelism between the axis of the feed rolls and that of the platen in order to insure proper and consistent paper feed. The spring biasing or other mechanical force exerted to cause the feed roller to engage the platen is such that it will provide the desired radial forces against the platen for proper paper feed, but may be unevenly exerted if the feed rolls are not positioned exactly parallel with the axis of the platen.